Neuromuscular junctions form in a tissue culture compartmental system in which two separate populations of cholinergic neurons innervate a common population of muscle cells. Stimulated and unstimulated afferents in unilaterally stimulated preparations lose connections equally, and bilaterally stimulated preparations lose more connections than do unilaterally stimulated ones. These effects contradict the Hebb model of activity-dependent synapse strengthening and elimination. Loss preferentially affects connections that are from neurons furthest away spatially from the innervated muscle. We postulate that spatial contiguity effects may be involved in many developmental synaptic remodeling phenomena. The kinetics of calcium increase, adaptation and recovery have been determined in stimulated sensory neurons. Only the initial accumulation kinetics change with chronic activation. A number of immediate early genes (IEGs) have been characterized with regard to their single cell distribution and responses to depolarizing stimuli. A high degree of selectivity for specific patterns of electrical activity has been demonstrated. Bursts are more effective than steady stimulation, but c-fos responds to 1/min but not 1/2 mins to bursts (same total number of action potentials) while Nur 77 responds equally well to either pattern. Northern blot analysis fails to show a change in GAP-43 transcription in sensory (DRG) neurons with electrical stimulation in either the absence or presence of synaptic targets for the DRG cells. This suggests phosphorylation or other post- transcriptional changes may be involved in any role this protein has in plasticity in the nervous system.